In an optical communication system, optical signals are encoded with digital streams of information and transmitted through a series of optical fiber spans towards a receiver end, where the optical signals may be decoded to retrieve or re-generate the digital streams of information in electronic domain. The encoding is typically performed by modulating the optical signals in phase, amplitude, or both. Mach-Zehnder (MZ) waveguide interferometers are often used to modulate optical signals in amplitude and/or phase.
Referring to FIG. 1, a prior-art optical modulator 100 may include first 101 (“X”) and second 102 (“Y”) modulation branches. Each modulation branch may include a pair of MZ waveguide interferometers 110 for in-phase (“I”) and in-quadrature (“Q”) modulation. First 111, second 112, and third 113 Y-splitters may be used to split an incoming optical signal 104 into four equal portions. First 121 and second 122 Y-combiners are used to recombine the optical signal 104 into an “output X” branch 131 (modulated light 105X) and an “output Y” branch 132 (modulated light 105Y) for coupling to an optical polarization rotator/combiner, not shown. The modulation configuration of the optical modulator 100 is termed a “Dual-Polarization, Quad Parallel Mach-Zehnder” (DP-QPMZ) modulation configuration.
One drawback of the prior-art DP-QPMZ optical modulator 100 is a comparatively large size, and associated high cost. Due to their geometry, the Y-splitters 111-113 and the Y-combiners 121, 122 of the optical modulator 100 typically occupy large area on a modulator chip. A requirement to have optical taps at different locations of the optical modulator 100 may result in a further size increase of the optical modulator 100.